The present invention relates to a slip control system, especially a traction control system, that reduces the torque of the drive unit of the vehicle when wheel slip becomes excessive at at least one driven wheel.
A traction control system is described in European Patent No. 386 126. According to this patent, the slip is determined at at least one driven wheel, and the engine torque is reduced as a function of this slip. If this driven wheel reverts to stable operation, i.e., there is no longer any unacceptable slip, the engine torque is slowly increased (ramp-up) by appropriate control of, for example, a throttle valve, according to a given function using variable parameters. This known method is problematic in that the traction control system is tuned for normal operation, especially with respect to the magnitude of the torque cutback and/or the steepness of the ramp-up. In special driving conditions, such as on rough sections of road, the traction control system tuned for normal operation demonstrates unsatisfactory dynamic characteristics in some application cases. In particular, deteriorated traction characteristics, which have a negative effect especially when the vehicle is to accelerate after or during driving over a rough section of road, occur as a result. Because the traction control system responds to rough sections of road when there are short-term wheel disturbances, the acceleration performance of the vehicle is impaired by the torque cutback and the slow ramp-up.
A comparable performance occurs in connection with an engine drag-torque control, in which there is an acceleration of the vehicle that is not desired by the driver during or after driving over a rough section of road. This is especially the case for a drag-torque control in which a controllable clutch is disengaged.
From the related art, there are various methods available for recognizing a rough section of road. For example, German Published Patent Application No. 42 15 938 (U.S. Pat. No. 5,357,788) describes the recognition of a rough section of road on the basis of the behavior of a wheel-speed signal. Described in German Published Patent Application No. 195 49 083 is a rough road recognition system based on an acceleration sensor, which is used, for example, in connection with restraint systems. In general, driving over rough sections of road causes wheel fluctuations, e.g., a fluctuation of the wheel speed above the slip threshold.
Due to the torque reduction being diminished or being of a shorter duration, especially due to the control of the magnitude of the torque reduction and/or the rapidity of the ramp-up, the traction of the vehicle on a rough section of road is substantially improved.
In particular, deteriorations in traction when there is a reacceleration after or while driving over a rough section of road, especially in a curve, are effectively prevented in this manner, since the effects of the traction control system on the traction of the vehicle are reduced.
The methods known in the related art are used in an especially beneficial manner for recognizing the rough section of road, so that the expense required to implement the described improvement is small.
Furthermore, the application of the aforementioned design approach is also beneficial when used in conjunction with engine drag-torque control systems (MSR), the increase in engine power being less and/or of shorter duration when a rough section of road is recognized, i.e., the cutback of the increase occurring more rapidly. In this way, unintended acceleration is prevented.
In a slip control system that disengages a controllable clutch in the deceleration operation in response to an incipient wheel lock or an incipient wheel spin at at least one driven wheel, the disengagement is completely or partially (setting a particular slip at the clutch) abandoned when a rough section of road has been recognized. This also improves the unsatisfactory performance of the vehicle in this situation.